Such contactless measuring devices are applied in many branches of industry, e.g. in manufacturing, in the chemical industry and in the foods industry.
In performing fill level measurements, short transmitted signals, e.g. of microwaves or ultrasonic waves, are periodically issued from an antenna onto the surface of a fill substance and their echo signals reflected on the surface are subsequently received, following a distance-dependent travel time. An echo function representing the echo amplitudes as a function of travel time is formed. Each value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the antenna.
From this echo function, a useful echo is determined, which probably corresponds to the reflection of a transmitted signal from the surface of the fill substance. In this connection, it is usually assumed that the useful echo is the one with the largest amplitude. From the travel time of the useful echo, in the case of a fixed propagation velocity of the transmitted pulse, the distance between the surface of the fill substance and the antenna is immediately obtained.
Usually, a raw, received signal is not used for evaluation, but rather its so-called “envelope curve”. The envelope curve is produced by rectifying and filtering the raw signal. For exact determination of a travel time of the useful echo, first a maximum of the envelope curve is determined.
This conventional way of proceeding functions in many applications without problem.
Problems occur, however, always when the echo coming from the fill level cannot be clearly identified. This can be the case, for instance, when installed objects are present in the container and they reflect the transmitted signal better than the surface of the fill substance.
In such cases, the actual fill level can be input into the fill level measuring device, e.g. at start-up. The fill level measuring device can then use the entered fill level to identify the associated echo as the useful echo and e.g. proceed further on the basis of a suitable algorithm. For instance, maxima of the echo signal, or the echo function, are determined in each measuring cycle, and the useful echo is determined on the basis of knowledge of the fill level determined in the previous measuring cycle and an application-specific maximum expected rate of change of the fill level. The signal travel time of the thus-determined, current useful echo is then used to ascertain the new fill level.
Such echo tracing has the disadvantage that the useful echo must be kept track of without interruption. If the fill level measuring device is taken out of service, e.g. for purposes of maintenance, then the useful echo, as a rule, gets lost. The fill level measuring device is then not able to recognize the useful echo reliably. It becomes necessary to start over by repeating the initial input of the current fill level. This can, however, entail, in certain cases, considerable expense. When, for instance, there are no alternative measuring methods available, it can be necessary to empty the container completely, or to fill it completely or to a reference mark. Industrially, this frequently means a shut-down of the manufacturing process and can be associated with considerable loss of time plus high costs.